Having developed a variety of techniques and methods for assessing hemodynamic and myocardial functions in unanesthetized rats, we propose to continue our studies using the spontaneously hypertensive rat as an experimental model of naturally developing hypertension. Thus, studies dealing with systemic and regional blood flows, cardiovascular and myocardial reflexive adjustments to elevated pressures, and regional flow distribution will be performed in the spontaneously hypertensive rat and two normotensive control groups (the American Wistar and the Wister-Kyoto). Systemic flows will be measured either by using the direct Fick method or by chronically implanting Doppler flow probes. Regional flows will be assessed by microspheres; and fluid partitions will be quantified by tracer techniques. Using these methods we also hope to learn cardiovascular responses to a variety of physiological and pharmacological interventions, including their respective effects on reflexive capabilities and responses to anesthetic, diuretic, and antihypertensive agents. Hemodynamic effects of stimulation and ablation of key brainstem nuclei also will be determined; and we hope to identify receptor presence and affinity to naturally occurring circulating vasoactive and cardioactive agents in heart and brain.